As part of our efforts in pediatric oncology and hematology, we plan to study the role of immune and coagulopathic mechanisms in platelet transfusion therapy. We will perform platelet antibody studies by the serotonin release and complement fixation methods, HLA typing and coagulation studies in patients receiving platelet and other types of transfusions. The results of these studies will be correlated with platelet unresponsiveness. This information will be used to search for donors for platelet transfusions among family members and unrelated individuals. In addition, in vitro platelet function studies will be performed on donor platelets and on recipients' platelets before and after infusion. We will attempt to correlate these with platelet rise and platelet survival, control of hemorrhage and other untoward reactions. These studies will be performed in order to maximize the benefit of platelet transfusion therapy in patients with the thrombocytopenia associated with malignant disease and to determine the effect of platelet transfusions on overall survival of patients with malignant disorders.